Seat occupancy sensors are nowadays widely used in automotive vehicles to provide a seat occupancy signal for various appliances, such as, e.g. a seat belt reminder, an auxiliary restraint system (airbag), etc. The possibility of customization and personalization of the vehicle by the customer is a key selling factor of modern cars. This leads to many different variants of car interiors being offered for one car model. With the increasing number of available options, severe constraints arise concerning the implementation of technical equipment in the vehicle. With seat occupancy sensors arranged between the foam body of the seat cushion and the seat cover, every seat design (leather, cloth, sport, comfort, . . . ) requires specific development effort for the occupant detection system. That induces high development costs and is, therefore, an unattractive solution for the automotive industry. Hence, a problem to be solved is to find a sensor solution, which is less influenced by seat design and thus can be used for a greater variety of car seats or even car platforms.
Document DE 197 52 976 A1 discloses a vehicle seat suspension mat, comprising an occupancy sensor in the shape of a film-type pressure sensor. The pressure sensor includes a first carrier film, a spacer and a second carrier film, which are disposed on one another in the manner of a sandwich. Contact elements are arranged on the inner surfaces of the carrier films. An opening in the spacer allows the contact elements to get into contact with each other when pressure is applied on the sensor. The pressure sensor is arranged inside a cavity on the bottom side of the foam cushion of the vehicle seat. The pressure sensor is supported by a foam block, which closes the cavity and which rests on the seat pan.
Document DE 20 2010 003 563 U1 discloses a vehicle seat suspension mat, comprising a film-type pressure sensor. The pressure sensor includes two films maintained at a distance by a spacer material arranged there between. The pressure sensor is disposed on a compressible intermediate layer, which is, in turn arranged on a base plate.
The solutions presented in DE 197 52 976 A1 and DE 20 2010 003 563 U1 use each an intermediate foam layer between the film-type pressure sensor and its support (i.e. the seat pan or the springs of the seat suspension mat) in order to allow the reaction force of the support to compress the film-type sensor from below. The activation threshold of the film-type pressure sensor (i.e. the pressure above with the film-type pressure sensor is activated) may be adjusted by appropriately selecting the compressibility of the intermediate layer.